The field of the invention is water treatment and the invention relates more particularly to the treatment of water used in evaporative recirculating cooling systems.
One such method is the chemical treatment program which relies on the use of sequestrants and inhibitors for both scale and corrosion control. The system is simple in that it may be controlled by adding the products directly from a container to the cooling water and the system is capable of maintaining a reasonable amount of control over scale and corrosion. However, in order to properly control the system, an increasing volume of water must be bled from the system to keep dissolved salts within their ranges of solubility, thus preventing scale formation. This requires large additions of make-up water to keep the concentration cycles within a scale-free range.
The second commonly used system is one in which the pH and alkalinity of the cooling water are controlled through the addition of pH adjusting chemicals. This type of system has the advantage of permitting somewhat higher concentrations of dissolved salts to build up in the cooling water, thus allowing greater xe2x80x9ccycles of concentration.xe2x80x9d This results in considerable savings of water as less make-up water is required as compared to the polymer treatment program. The chemical additions are capable of scale and corrosion prevention. This maintains cleaner and more efficient heat transfer surfaces.
An improved system is shown in U.S. Pat. No. 5,730,879 of which applicant is a co-inventor. This process utilized a strong cation exchange unit which operated on a side stream of the recirculated evaporative cooling water stream. While this system was generally satisfactory, it raised a problem when regeneration of the strong cation exchange resin was necessary. This problem related to the disposal of the regeneration solution since the resin may have picked up toxic chemicals from the recirculated evaporative cooling water stream. Thus, the cost of regenerating the units became expensive and difficult.
Evaporative cooling systems, of which cooling towers are one example, operate on the principle that the latent heat of vaporization of the water being evaporated removes energy from the system, thus, reducing the temperature of the remaining water in the system only some of the water is evaporated, and the salts in the remaining water are concentrated. This results in increasing the amount of dissolved solids in the recirculating stream. The most common dissolved salts in domestic water are bicarbonates, chlorides, and sulfates of calcium, magnesium, and sodium. When a water containing calcium bicarbonate is heated, as in cooling of air conditioning systems, or other equipment, the heat will strip off one molecule of carbon dioxide, converting the remaining calcium salt to calcium carbonate (limestone) according to the equation:
Ca[HCO3]2+[heat]xe2x86x92CaCO3↓+CO2+H2O
Unlike most salts, calcium carbonate is less soluble in hot water than in cold water. As a result, scale (calcium carbonate scale) is deposited on heat exchange surfaces. Calcium carbonate solubility is also a function of the pH of the water. Calcium salts are markedly less soluble in high pH systems. Knowing these properties of dissolved solids in cooling waters thus offers several means of control. Controlling the pH will allow more calcium to remain in solution, thus preventing it from becoming a hard scale on a heat exchange surface, or perhaps better still, if one were to eliminate calcium and magnesium from the system entirely, or at least drastically reduce them, the same control could be exercised. This has, in fact, been done through the provision of a water softener to soften the cooling water (i.e. remove the hardness causing ions, calcium and magnesium). While this effectively controls scale, it does not necessarily eliminate the need for acid feed to control pH. The water softening approach also requires extremely large equipment to soften water and is, thus, impractical. Furthermore, softened waters tend to be more corrosive than unsoftened ones. Thus, in these systems, the scaling problem may have been eliminated but at the expense of increased corrosion due to increased salts.
Several patents discuss various approaches to treating make-up water, including U.S. Pat. Nos. 2,807,582; 3,805,880; 4,532,045; 4,931,187; and 5,145,585.
In order to be practical on a wide scale, a water treatment system must be simple and capable of use by plant personnel.
It is an object of the present invention to provide a cooling water treatment process capable of maintaining absolute scale and corrosion free heat transfer surfaces and of reducing water consumption while also eliminating difficulties in regenerating resins used with the process.
The present invention is for a process for conditioning recirculating evaporative cooling water and/or make-up water which includes the steps of determining the pH of saturation. Also, the pH of the recirculated evaporative cooling water stream is measured and its difference from that of the pH of saturation is determined. Based upon this difference, a side stream of the source of make-up water is diverted into either a weak cation exchange resin or a strong cation exchange resin, or a combination of both, and returned to the make-up water.